Radiophysics. Халюшева Г.Р. - 19 стр.

         4 Unit 4
         4.1 Text 4
                                                      How sounds are produced?

         Sounds are produced by the vibrations of matter. If sound vibrations are regular, they are pleasant to the ear and are
known as musical sounds.
         One can make matter vibrate in different ways. You can strike wires with a hammer. You can set air in vibration
with your lips, your can even make gases vibrate. When you wave your hands back and forth they are vibrating in a way.
However no sound is heard because the vibrations are not fast enough.
         A dog can respond to sounds that you cannot hear because its ears are sensitive to higher rates of vibration than you
can receive.
       A vibrating object makes the air around it vibrate. When a body vibrates, it sets up
a wave motion in the surrounding air. The waves are carried in all directions from the
vibrating body. They move as water waves move away from the spot where you have
thrown a stone into the water. The difference between these two types of waves is that
sound waves travel only on the surface of the water.
           The loudness of a sound depends on the amount of matter that vibrates. The waves are higher in water if you drop a
large stone because more water is suddenly pushed aside. In the same way, the more air object causes to vibrate, the louder
will be the sound. Loudness is due to amplitude; the amplitude is the height of the wave from the center to the top or bottom
part of the wave. As the height of the wave becomes less as it moves away, so the loudness of sound decreases as you move
away from the source. When sounds are made louder, as in a radio amplifier, the wave amplitude is increased. This is done
by using a greater amount of energy to produce the sound vibrations.
           The number of times an object vibrates per second is its frequency of vibration. The frequency of vibration
determines the pitch of a sound. By pitch we mean how high or low the sound is. If you strike a tuning fork against your
hand or knee, its prongs will vibrate 256 times a second. No matter how hard you hit the fork, it will still vibrate 256 times a
second. The pitch of the tone will stay the same. What did you change when you hit the fork harder? How might you change
the pitch of the tuning fork?
           A wave length is the distance from the top of one wave to the top of the next succeeding wave. When you strike a
tuning fork, the rate of vibration (frequency) and the wave length are constant. The amplitude of loudness of the sound
depends on the force of the blow.
           Sound waves are usually carried to our ears by the air. Have you heard the shot of a gun fired at quite a distance
from you? How did sound waves get from the gun to your ears? You probably have noticed that when you strike two stones
against each other under water, you can hear the sounds produced under water.
           Gases, liquids, and solids carry sound waves. Sound travels in air at a speed of approximately 1, 100 feet per
second. When you see a flash of lightning a mile away, the speed of light is such that the flash reaches your eye almost
instantly. The sound of the resulting thunder reaches your ear later. By using an accurate stop watch, you can measure the
interval between flash and thunder.
           Sound waves travel faster in water, and still faster in solids such as wood, stone, and iron. When the temperature of
air rises, the speed of sound increases. Sound travels slower at high altitudes than it does near the earth’s surface. Why?
           Sound waves can be reflected. You no doubt have noticed that when you shout toward a distant wall, the sound is
reflected back to you; we call sounds reflected in this way echoes.
           The ear cannot distinguish between direct sound and its reflected sound if they are less than one–tenth of a second
apart. Sounds in small rooms are reflected back in time to be mixed with the original sound, and thus there is usually no
noticeable echo. If the room is more than 55 feet long, echoes are produced which interfere with the original sounds.
           Auditoriums are now built with rounded corners and few large flat surfaces. This prevents sound waves from being
reflected to any position. They are scattered in many directions and the only sounds heard are those sent out from the
source. Many materials are made which absorb vibrations or break up the waves. Some fiber–boards having many holes are
used to soundproof rooms. Again the waves are either absorbed or scattered so there is very little reflection.
           We find today that in modern buildings the architect uses methods and materials which reduce echoes and favor
good sound transmission.

         4.2 Words and word–combinations to be remembered
         Vibration – вибрация
         to vibrate – вибрировать; колебаться